Acoustic electric energy converter



Feb. 25, 1936. G. w: PIERCE 2,031,789

' ACoUs-TIC ELECTRIC ENERGY CONVERTER Filed Aug. 9, 1952 f Invenoz G6074 e Zz). fierce A orney Patented Feb. A25, 1936 2,031,189 ACOUSTIC ELEC'rmo ENERGY CoNvEn'raa George Washington Pierce, Cambridge, Mass.

Application August 9, 1932, serial Nn. 62u42 o 9 Claims.

The present invention relates to devices :l'or

converting or translating acoustic into electric energy and vice versa, such as loud speakers, and

more particularly to devices of this character operable by magnetostrictive action.

A chiefobject of the invention is to provide a loudspeaker operable by magnetostrictive action. Other objects will be explained hereinafter, and will be particularly pointed out in the appended claims.

The term loud speaker is used herein to denote a device for delivering sound with suinciently faithful quality and sufficient intensity to be heard throughout a room. The device of the present invention yields acoustic energy of essentially the same magnitude as the other types of loud speakers now on the' market and the quality may be made essentially as good.

It will, therefore, be understood that the term loud speaker", as used herein, is not properly applicable to an ordinary telephone receiver that is designed to be held to the ear. A telephone receiver, if driven with enough power to raise the intensity to loud-speaker proportions, is defective o in quality. It has heretofore been proposed to A magnetostrictive vibrator resonator com-l prises a magnetostrictive core disposed 'in an electromagnetic eld, such as may be established by passing an electric current through a field coll or winding. 'I'he core may be in the form of a rod or tube, or any other desired form. Any

ferro-magnetic or other metal having suitable properties may be used for the core, but the material should obviously be characterized-by com- -paratively large or strong magnetostrictive effects or qualities and comparatively low vibrar tional decrement. The core may be constituted of a bimetallic bar (not shown) made up of two bars of diiferent magnetostrictive materials and welded together throughout their length along their adjoining surfaces, as described in Letters Patent of the United States No. 1,882,399, grant- 00 ed oct. 11, 1932.

' When eaimulated magnetically by the field, the core becomes very slightly mechanically deformed or distorted by magnetostriction. The resulting increment of deformation may be a lengthening expansion or a shortening contraction, or some (Cl. 11s-11o) i other distortion, depending on the material and on the polarity ofthe increment of the magnetic field. 'I'his action of the magnetic field uponfthe core will, for brevity, be hereinafter termed stimulation. Conversely, when the vibrator is mechanically deformed .or distorted, it will react magnetically uponV the magnetic field by magnetostriction, with an increment of magnetization depending upon the nature of thepreexlsting magnetic field and the mechanical deformation, and this will produce `its effect upon the electric current or voltage in the coil. This reaction will, for brevity, behereinafter referred to as the response.

If the current or voltage is alternating, the electromagnetic field created thereby will also be alternating. The core will, therefore, increase and decrease in length, let us say, many times a second, every variation in the current producing its stimulative effect on the core,- andl every deformation of the core producing its reaction response upon the current. The core will, in consequence, vibrate mechanically by magnetostriction. Ordinarily, these vibrations will be quite small. When the alternating frequency is varied so as to assume a value close to, or substantially the same as, the natural frequency of mechanical vibration of the vibratory system, however, the amplitude of vibration of the core, though still small, becomes relatively quite large. The core will then react inductively on the load to render its consumption of power critical as to frequency for frequencies near the free frequency of the system.

The invention will be explained in greater detail in connection with the accompanying drawing, in which Fig. 1 is a diagrammatic view of circuits and apparatus arranged and constructed in accordance with a preferred embodiment of the present invention, illustrating a loud speaker: and Fig. 2 is a similarview of a modification.

- A magnetostrictive core 2, longer than it is wide, is rigidly secured, as by integral attachment,

, at its lower end, to a base 13, Fig. 1, which may be in the form of a mass having substantial inertia. 'I'he magnetostrictive core is axially positioned within, and driven by, a solenoid field coil or helical electromagnetic winding 24, with clearance to permit free vibrations. The winding 2l is arranged longitudinally of, and around, the core' 2 and encircles it for the greater part of its length, as illustrated. For loud-speaker or other reception or sound-reproducer purposes, the coil ZI may be connected in the alternating-current Qutput circuit of a suitable, audible-frequency,

ycoil of which is indicated at 42.

. series with a local battery I8. The gridy 32 and the lament 26 are shown inv circuit with the secondary coillli of a transformer, the. primary The tube 3l) f constitutes a source of alternating electromotive force or current for the solenoid 24, to enable the latter to drive the magnetostrictive core 2.` The winding 24 may-be lfed at, or substantially at, the natural frequency of the system, as before explained, to get lthe most pronounced eiect.

The battery I8 may serve also to apply a steady magnetizing or polarizing iield to the core 2, over which the alternating iield produced by the generator 3U is superposed. The alternating iield is preferably smaller than the steady field, in order that the combined iield may not at any time fall to zero. Polarization may be eifected in other ways also, as described in the said Letters Patent of the United States No. 1,882,401, and in Letters Patent of the United States No. 1,750,124, granted March 11, 1930, where the theory of the operation is also described in greater detail. Thus, the battery may be dispensed with, and the core may be magnetized electromagnetically by a local source sending a polarizing current through a separate polarizing'coil, or it may be permanently magnetized, instead, if the vibrator has sucient magnetic retentiveness, or the-battery and a permanently magnetized rod may be employed together. In particular, the magnetization or polarization of the core 2,-may be produced by a strong, permanent magnet of substantially constant, relatively large or intense magnetic eld of predetermined strength, supported near the vibrator, ashereinafter described, and it may have magnetostrictive qualities oppositeto those of the core 2.

The operation will be better understood by the following example: Nickel is a magnetostrictive material that shortens with an increase of magnetization. The shortening or static magnetization is very minute. If, however,.it is polarized by' an applied steady field, upon which is superposed an alternating current resonant with its natural frequency of mechanical vibration, it becomes specifically treated to increase its lineal or other magnetostrictive displacement, and its amplitude of length variation may become sevveral hundred times as great as withl static or nonresonant magnetization. Y

The upper end of the core 2 is shown connected with a loud-speaker, sound-radiating member or diaphragm 68, or with some other sound-radiating or receiving member, as will be understood by persons skilled in the art.

According to the modification of Fig. 2, the radiating diaphragm 68 may, for example, be constituted of a. wooden boardor a metal sheet, attached to any desired support. Thesaid upper end of the core 2 is shown attachedto the diaphragm 6B by a crow foot or spider 15. The spider 15 serves to extend the attachment of the core'to a larger area. of the diaphragm 68, which permits a better path for transmission of sound vibrations. vThe function of the weighted mass 13 is described in the said Letters Patent 1,882,401.

According to the modiication of Fig. 2furtherrnfwe, permanent magnets or ferro-magnetic magnetostrictive variations. A lever bars 9| are attached at one end, as by means of bolts 93, to the inertia member 13 and-extend upwardly from their attaching points throughout substantially the length of the core 2. The crow foot or spider 15 may then preferably be made of iron, and small air gaps may be provided between the endsof the spider and the ends of the bars 9| to permit free vibration of the radiating face 68.

In Fig. 1, the core 2 and weighted' end are substantially as before described, but the winding 24 is surrounded by, and arranged within the magnetic field of, an elongated frame member or housing 91. As the housing 91 is double-U- shaped in cross section, it encircles, and is spaced v slightly from,.and longitudinally of, the core 2. The housing 91 may, if desired, be a magnet or be constitutedof magnetic material, and be used for polarizing the core 2. The opposite poles of the magnet are disposed opposite to the ends of the core 2, the pole at the lower limb |04 of the magnet 91 being in magnetic contact with the 'core 2 at its lower, rigidly fixed extremity and the pole at the upper limb |06 of the magnet 91 being in close proximity to, but out of engagement with, and encircling, the core 2, as illustrated. A return magneticv pathv is thus provided for the magnetic flux in the core 2 with a minimum air gap. The magnet should obviously be of such strength as to premagnetize the core 2,-to that pointy on they magnetostrictive core 2 corresponding to itsI `most eiiicient point of sensitivity to 8 is pivoted to the frame 91 at 99 and bears upon the free end of the core 2. The arms of the lever 98 are unequal in length, as shown. The core impedance is vthus so adapted to the diaphragm impedance as to make possible the use of a less massive diaphragm.

Variations in the current in the elcctromagnet winding 2l will react upon the magnetic eld to cause undulatory changes in the magnetic ux of the core 2, thus producing correspondingly opposite magnetostrictive-eft'ect variations or changes in the lineal or other dimensions of the core 2 and in the winding 24, cumulative in their effect to reproduce voice sounds by vibrating the diaphragm 68. At, or substantially at, the natural frequency of the system, the reaction of the magnet 91 on the core 2 is periodically varied to a maximum extent, as before described, so as to produce a maximum effect-in the loud speaker. The mass attached to the non-radiating end of the core may be about ten pounds. The radiator may be a plane wooden board, about 1 to 2 inches thick, preferably more than one inch thick, and may have an area of 8 to 100 or more square feet, but other shapes of radiating members may be used, as before described, characterized by graduated stiffness, density or speed. Where ithe radiator is curved or in the form` of a cone, vthe thickness should correspond to the increase of stiffness of the curved or conical surface.

The invention is not, of course, limited to the modifications illustrated herein, and may b e further modified and changed by persons skilled in the art without departing` from its spirit and scope, as dened in the appended claims. What is claimed is: 1. In a sound reproducer, a metallic rod having strong magnetostrictive qualities, an electro-magnetic coil encircling said rod and spaced slightly therefrom, a strong permanent magnet arranged with opposite poles thereof opposite different ends oi' the rod, a clamping means rigid- 'I5 aesinet lysecm'ingmeendoftherod,andadiaphragm eonnectedtothleon'positeendoi'therod,said` coil when connected to alternating current being eiiective to vibrate the diaphragm in accordance with the current.

2. In a sound reproducer. a vibratory member, an actuating element i'or the member constructed of ferro-magnetic metal having strong' magnetostrictive qualities. said element being rigidly secured at one end and being attached at the other end to the vibratory member. a permanent magnet of relatively large and predetermined strength arranged longitudinally to said element,

said magnet. premagnetizing said element substantially to its msust eilicient point of sensitivity to-magnetostrictive variations, and a helix or electromagnetic coil encircling said element for the greater part of its length, said helix being adaptedtobeconnectedtoasomceofalternating current to cause corresponding lineal ex.

pension and contraction of said element, thereby to vibrate said member.

3. In al sound reproducer, a ferro-magnetic member. an elongated U-shaped permanent mag-l net ot predetermined strength. said member belng longitudinally supported in the magnetic neld .of said permanent magnet, one pole of the magnet being in Vmagnetic contact with the lmember atY one rigidly ilxed extremity. the other pole being in close proximity but out oi engagement with the'member, thereby providing a return path for magnetic iiux in the member with minimum air sap, a diaphragm connected to one end of the member, and an electromagnetic coil associated with the member connected with the output ot a suitable audible-frequency sound amplifier, said electromagnetic coil causing unduiations oi' the magnetic ilux in said member and corresponding changesin lineal dimensions of the member. thereby'to reproduce voice frequency sounds by vibrating the'diaphragm. 1

4. In a sound reproducer, aldiaphragm. a

"manentmaghetanoperatingmemberiorthe diaphragm o! ierromagnetic material, said member being rigidly supported and in magnetic com' tact with one limb of the magnet and having its -freeendencircledbytheotherlimbofsaidmagnet,andengagingwith thediaphragm,andan electromagnetic coll arranged longitudinally 4to both'magnet and member and connected to-a source ot variable current, variations in current in the electromagnetic coil causing magnetostrictive eifects in the magnet to produce an eiiect on the diaphragm.l

5. Ina sound reproducer, a metallic member v rigidly secured at one end and attached to a diaphragm at its other endr a magnet arranged longitudinally thereoi' magnetizing the member.

and a helix around said member, said helix being adapted to be traversed by alternating currenty which causes changes in the magnetic ilux in said member to cause corresponding linear exvacuum-tube amplifier, and a soundradiating element coupled to one end of the member, said electromagnetic coil causing undulations in the vmagnetic ilux of said member to cause correspending changes in the dimensions oi said member, thereby to operate the sound-,radiating element to emit sound.

'1. In a sound reproducer. a diaphragm, a metallic member coupled thereto longer than its own width, a magnet for premagnetizing the member substantially to its highest point of sensitivity to magnetostrictive variation, and an electromagnetic coil for causing changes in the magnetism ot said member, thereby toproduce changes in length oi the member to vibrate the diaphragm.

8.' An electro-mechanical resonator operating by'magnetostriction comprising in combination ja magnet having a substantially constantyrelatively intense magnetic field, a magnetostrictive vibrator" arranged within said neld and a coil adapted to be fed, with alternating current at or substantially at vthe natural frequency of said vibrator, whereby alternating current ilowing in, the coil acts in conjunction. with the magnetic. v

field to produce magnetostrictive variations in Y-the dimensions oi' the vibrator.

9. An electromechanical resonator as claimed inclalmandinwhlchthecilfedwithalterr nating current at or substantially at the natural frequency oi' the vibrator, Vsux-rounds saidV vibrator as described. 

